Purple liquid[1], also known as hazardous liquid[2], dangerous liquid[3], and deadly liquid[4][5], is a corrosive substance in Metroid II: Return of Samus and Metroid: Samus Returns. It blocks off areas on planet SR388 and must be drained by defeating all Metroids in the current area in order to continue exploring. The titular creatures do not live in rooms where the liquid is present.

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In Return of Samus, the liquid decreases during an earthquake, the latter which is triggered when Samus Aran kills all of the Metroids in a specific area as part of her mission. The game offers no explanation on how and why their deaths trigger or coincide with SR388's earthquakes.

The liquid is slightly opaque compared to water, the other liquid seen in the game.

In the remake of Metroid II, Samus must kill all of the Metroids in a given area and collect their DNA in order to upload it into a Chozo Seal that drains the liquid from the corresponding location. The substance is corrosive to the point of leaving a dark purple taint on the surfaces of tunnels the liquid used to occupy. It is very harmful to Samus even in her Gravity Suit, yet it seemingly poses no threat to all native life on SR388 with the exception of Metroids. Indeed, Gawrons are seen emerging from Air Holes inside the liquid, Halzyns can land inside the liquid and emerge unharmed, and many animals such as Motos and Hornoads are seen roaming the very caverns where the hazardous liquid was once present mere moments ago. However, enemies do not appear inside the depths of the liquid, so the latter's presence can alternatively be explained by the creatures simply exploring their newly accessible environment every time Samus removes the purple substance, similar to how they suddenly repopulate rooms after she kills the resident Metroids.

The purple liquid has a unique characteristic of not triggering a Reserve Tank if one was unlocked with the Zero Suit SamusSuper Smash Bros.amiibo. Even if Samus has the Reserve Tank active and charged, it will not work if her energy is fully depleted by the liquid.

Metroid: Samus Returns Official Guide and image 9/11 of the Chozo Memories reveal that the liquid was used by the Chozo against their out-of-control creations, the Metroids. The purple substance is seemingly harmful to Metroids as they tend to be found in far proximity from it, successfully keeping them at bay deep underground within the main areas. Its harmful effect on non-native lifeforms is also of intentional design, as it is meant to discourage and/or prevent off-world visitors from exploring the planet. Thus, the substance not only protects visitors from certain death against Metroids, but it also keeps the species from falling into malevolent hands.

Unfortunately, the implementation of the substance was ultimately flawed. Despite the Chozo flooding their own cities and massive portions of the planet with the liquid, an untold number of Metroid larvae managed to reach or remain on the Surface of SR388 regardless, thus allowing the species' eventual discovery by the Galactic Federation. This led to the events of Samus Aran's Zero Mission and the entire history of the creatures spreading across the cosmos.

Prior to the release of Samus Returns, it was commonly believed and occasionally implied that this substance was either lava or acid. However, this was impossible to tell due to Return of Samus lacking the necessary color palette, as it plays on the Game Boy and as such, is completely in black and white or shades of green along with the Game Boy Player coloring it blue. The instruction manuals for Metroid II (both English and Japanese) only referred to it as a "dangerous liquid", but depicted it as red, like lava. The Super Game Boy Player's Guide further complicates the matter by initially describing the deadly liquid as poisonous acid, but then later refers to "glowing lava" while showing a screenshot of the liquid (with a reddish color for the specific color scheme 1972-1579-8071) in Phase 8.

Metroid Fusion seemed to support the idea that the substance was lava, given the fact that there was a room in the SR388 replication sector of the B.S.L filled with it. However, it was revealed in Samus Returns that both lava and purple liquid existed on the planet and were not the same.

Octrolls thrive in this substance. A Japanese commercial depicts an Octroll emerging from an inexplicibly blue and black liquid. They are not present in Samus Returns.

"The tunnels between each area are filled with a dangerous liquid that will deplete Samus of her energy. When an earthquake occurs, the liquid may drain away or fill in areas of the tunnels. Whatch [sic] out for the liquid! You must learn to work around it."

"Some parts of SR388's underground labyrinth are inaccessible because of the dangerous, energy-sapping liquid that floods them. When an earthquake occurs, that liquid shifts, emptying some areas and filling others. You must learn to work around it."

"This liquid doesn't deal quite as much damage as lava, but it causes a lot of problems if you stay in it too long. It serves as a gate between areas, its purpose to keep the Metroids from reproducing, escaping, or being taken by those who are up to no good — like Proteus Ridley, Mother Brain, and the rest of the Space Pirates."

The apparent lack of purple liquid between Area 8 and the Surface of SR388 is potentially the reason why Metroid larvae were able to reach the latter area after the release of the purple liquid, leading to their discovery.

However, the Chozo Memories do not offer details on the amount of time it took for the bird race to create both the purple liquid and Chozo Seals after the Metroids grew out of their control. This leads to the possibility that younger Metroids may have simply escaped SR388's cave systems before the flooding took place.

Despite Metroid: Samus Returns Official Guide stating that the purple liquid's purpose is to keep Metroids from reproducing, the Queen Metroid was nowhere near the last known location of the harmful substance. As previously mentioned, there is also no presence of said liquid, nor any evidence that it may have once been present between her nursery and the Surface of SR388. Therefore, the Queen would have been well capable of producing Metroid Eggs without hindrance, and her offspring would be able to freely move around in the two areas.

In Metroid II: Return of Samus, if Samus has collected sufficient Energy Tanks, she can dive into the hazardous substance for an extended period of time and reach other areas of the game in a non-sequential order. Reaching these earlier than intended reveals that the substance usually halts near the area's entrances, allowing Samus to emerge from the liquid, explore the locations safely and recharge her health with pickups dropped from enemies she finds and slays. This can be considered a sequence break as it allows the player to move to new areas without killing any resident Metroids.

The final area, where the Queen Metroid resides, is the only one in the game of which's entrance is entirely sealed off by a spiked structure, and Samus does not have enough time to make it back to the previous Phase 8 before the harmful substance kills her, even if she carries the maximum amount of Energy Tanks. The spiked structure disappears following the earthquake triggered by the death of the final Omega Metroid.

Although Metroids do not live in rooms where purple liquid is present, the baby can still safely move through these rooms without touching it. This may indicate that they simply prefer to live in less toxic areas.

In Samus Returns, M2 Ancient Chozo Ruins Arr plays in caverns currently flooded with purple liquid. The Matad Ambi theme plays in purple liquid caverns after the substance has been removed.